Re: shell variable references - coding style

[Jim Meyering <jim@meyering.net>]

On Tue, Feb 19, 2019 at 10:02 AM Bruno Haible <bruno@clisp.org> wrote:
>
> Hi Pavel,
>
> > > [...]
> > > This patch fixes both issues, and makes the IFS handling a bit more robust.
> > > [...]
> >
> > > -    case $_fpf_arg in
> > > +    case "$_fpf_arg" in
> > > [...]
> > > -  fpf_dirs=$1 ; shift
> > > -  fpf_cb=$1 ; shift
> > > +  fpf_dirs="$1"; shift
> > > +  fpf_cb="$1"; shift
> > > [...]
> >
> > ... and so on, I don't think it is more robust.  At least according to
> > Autoconf's Shellology [1] it should be actually better to write it the
> > other way around (without additional quotes).
>
> I do think it is more robust, because
>   * The number one mistake in shell scripts (measured by frequency of
>     occurrence) is to reference variables without double quotes when word
>     splitting is in fact undesired.
>   * Simple rules are easier to follow by programmers, resulting in fewer bugs.
>   * The rule
>       "Always double-quote shell variable references, except if you DO
>        want word-splitting."
>     is simpler than
>       "Always double-quote shell variable references, except if you DO
>        want word-splitting OR in the right-hand side of assignments OR
>        as argument of 'case' statements."
>
> The text that you quote says two different things:
>   * Backquotes inside double-quotes are hairy.
>     There is a simple rule to avoid them: When you have a backquote
>     expression, always first assign its result to a variable. Then use
>     the variable (with double-quotes, usually).
>   * Bash 4.1 has a bug when you WANT word splitting.
> These two things don't make my style rule
>   "Always double-quote shell variable references, except if you DO
>    want word-splitting."
> less robust.
>
> > FTR, Gary Vaughan has wrote a syntax checker rules for protecting us from
> > adding such statements into libtool codebase.
>
> Opinions regarding coding style differ. Gary is entitled to his opinion, as
> much as I am entitled to mine.
>
> Bruno
>
> > [1] https://www.gnu.org/software/autoconf/manual/autoconf-2.69/html_node/
> > Shell-Substitutions.html

I too prefer to avoid double quotes after assignment-"=": less syntax
feels slightly more maintainable, here.
These days I rely heavily on shellcheck. I find it to be very useful,
and at least by default it doesn't complain about such usage.